test(pdf-renderer): inject libLoader into createPdfRenderer to eliminate vi.mock factories

Adds an optional LibLoader parameter (defaults to the real pdfjs-dist dynamic
imports) and a failing test that verified the loader is called during init().
This is the first step toward removing ManualMockedModule registrations that
race with vitest-browser-playwright's birpc teardown (#535).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts

@@ -1,6 +1,11 @@
 import type { PDFDocumentProxy, RenderTask } from 'pdfjs-dist';
 
-export function createPdfRenderer() {
+type LibLoader = () => Promise<readonly [typeof import('pdfjs-dist'), { default: string }]>;
+
+const defaultLibLoader: LibLoader = () =>
+	Promise.all([import('pdfjs-dist'), import('pdfjs-dist/build/pdf.worker.min.mjs?url')]);
+
+export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) {
 	// Reactive state — exposed via getters
 	let currentPage = $state(1);
 	let totalPages = $state(0);
@@ -18,10 +23,7 @@ export function createPdfRenderer() {
 	let pdfjsLib: typeof import('pdfjs-dist') | null = null;
 
 	async function init(): Promise<void> {
-		const [lib, { default: workerUrl }] = await Promise.all([
-			import('pdfjs-dist'),
-			import('pdfjs-dist/build/pdf.worker.min.mjs?url')
-		]);
+		const [lib, { default: workerUrl }] = await libLoader();
 		lib.GlobalWorkerOptions.workerSrc = workerUrl;
 		pdfjsLib = lib;
 		pdfjsReady = true;